One of the problems affecting communication within a cellular telecommunications system is that of the interference generated by other communications of the cell or neighboring cells. A distinction is traditionally made between intercellular interference due to communication from neighboring cells and intra-cellular interference due to communications by the same cell where the terminal is located.
Many techniques have been proposed and implemented to reduce intra-cellular interference. Most of these techniques are based on an allocation of orthogonal transmission resources, for example time transmission intervals (TDMA), frequency transmission intervals (FDMA), OFDM orthogonal frequency-division multiplexing intervals (OFDMA), transmission codes (CDMA), transmission bundles (SDMA), or even a combination of such resources, so as to separate the different communications of a same cell.
Transmission resources being rare, they are generally reused, at least in part, from one cell to the next. A radio resource management (RRM) module is then responsible for statically or dynamically allocating the radio resources to the different cells. It is in particular known to statically reuse radio frequencies following a bi-dimensional pattern (Frequency Reuse Pattern).
This transmission resource management is, however, somewhat ineffective in high-density networks, heterogeneous networks or M2M (machine to machine) networks. A heterogeneous network refers to the superposition of a first cellular network with a low spatial granularity with at least one second cellular network with a high spatial granularity (made up of femtocells or picocells). The first cellular network is then called macrocellular, as opposed to the second network.
The allocation of orthogonal resources in the aforementioned networks would in fact result in an insufficient use of those resources, and at a low spectral efficiency. As a result, communications relative to users belonging to neighboring cells, or cells with different hierarchical levels in a heterogeneous network, generally experience in-band interference.
For a given communication, here called first communication, the interference caused by a second communication using the same transmission resource as the first is commonly called intra-band interference. In contrast, the interference caused by a second communication using a separate transmission resource (for example, a neighboring transmission frequency or another transmission interval) from that used by the first is called inter-band interference.
A network in which inter-band interference is predominant relative to the thermal noise is called “interference limited network” inasmuch as the capacities of different links of the network are more constrained by the interference than by the noise itself.
The treatment and production of inter-band interference have been the subject of considerable research.
The simplest processing method is to consider the interference as a simple thermal noise. This processing method is only acceptable, however, if the interference level is low. It should be noted that most power allocation algorithms are based on this hypothesis.
Other processing methods make it possible to reduce the interference by estimating the information signal of the interfering communication(s). This assumes that the considered receiving terminal knows the codes having been used to encode them. Known amongst these methods are in particular PIC (Parallel Interference Canceller) or serial (Successive Interference Canceller) interference reduction schemes, well known by those skilled in the art.
Another traditional approach for reducing the interference level is to implement an adaptive power control method. Such a method makes it possible to monitor the power levels of the different transmitting terminals so as to guarantee a predetermined service quality to the different users. This service quality can be measured depending on the case in terms of rate, latency, packet error rates, spatial coverage, etc. Traditionally, service quality metric refers to the parameter(s) used to measure it. As a general rule, a user's communication requires a minimum service quality that is taken into account or negotiated during the procedure to admit the user into the cell. This minimum service quality is expressed in the form of a stress on the service quality metric: latency below a threshold, rate greater than a guaranteed minimum, etc. The power allocation is then done so as to comply with the constraint on the service quality metric.
The power allocation methods generally adopt the hypothesis that the interference is comparable to thermal noise. However, this hypothesis is quite often pessimistic, such that the allocated transmission powers may be substantially greater than those necessary to obtain the desired quality of service. This allocation consequently leads to needless energy consumption and, for upstream communications, a reduction in the autonomy of the terminals.
Applications FR-A-2,963,194 and FR-A-2,963,195 recently proposed a centralized or distributed power allocation method, with a constraint on the service quality. More specifically, for a given constraint on the service quality, this power allocation method makes it possible to reduce the transmission powers of the terminals taking certain interference regimes affecting the different communications into account. Thus, according to this method, if a first communication is considered between a first transmitting terminal and a first receiving terminal, interfered with by a second communication between a second transmitting terminal and a second receiving terminal, three possible interference regimes exist for the first communication: a first regime with a high SINR (signal-to-noise and interference ratio), in which the first receiver processes the signal of the second communication as thermal noise, a second regime with a moderate SINR, in which the first receiver jointly decodes the information signals from the first and second communications, and lastly a third regime with a low SINR, in which the first receiver first decodes the information signal of the second communication, subtracts its contribution to the received signal before decoding the information signal of the second communication from the signal thus obtained.
This power allocation method works well for a pair of interfering communications. However, for a larger number of interfering communications, the situation becomes substantially more complex. It is in fact understood that a power allocation to a given transmitter affects the interference regime of the other communications and may modify their respective interference regimes. Thus, the power modification of one transmitter may lead to a power modification of one or several other transmitters. The power allocation may become unstable and diverge until reaching a situation where all of the concerned transmitters transmit at maximum power.
The aim of the present invention is to propose a link optimization method in a wireless telecommunications network, without modifying the power allocation of the different transmitters and therefore without any significant disruption of the interference situation between the different communications.